Synthesis of a Large Amino-Phenolic Cage. Synthesis, Crystal Structures, and Acid-Base and Coordination Behavior toward Cations and Anions

The synthesis and characterization of the new polyaza-phenolic-macrobicycle 32-hydroxy-1,4,7,10,13,16,19,22-octaazatricyclo-[11.11.7.126,30]-diatriconta-26,28,¢30,32-triene (L) are reported. L incorporates a 2,6-dimethyl-phenolic unit bridging two opposite amine functions of the [24]aneN8 polyazamacrocyclic base to obtain a large cage. The basicity and binding properties of L toward Cu(II), Zn(II), and Cl- were determined by means of potentiometric measurements in aqueous solution (298.1 ± 0.1 K, I ) 0.15 mol dm-3). L can add up to six acidic protons, yielding the H5L5+ species or the H6L6+ species, depending on the ionic medium used. The molecular topology of L permits the formation of a highly positive three-dimensional cavity in the polyprotonated species that is able to host the chloride anion. This was detected both using potentiometric data, log K ) 41.33 for the reaction L + 6H+ + Cl- = H6LCl5+, and in 35Cl NMR experiments that showed interactions also with the H5L5+ and H4L4+ species. The anion is probably hosted inside the three-dimensional cavity of L, and stabilized by H-bonding interactions with the ammonium groups, as depicted in the crystal structure of the H6L6+ cation reported. L forms mono- and dinuclear complexes with all the metal ions investigated; the dinuclear species are the only existing species with an L:M(II) molar ratio of 1:2 at pH higher than 6. The phenolate oxygen atom coordinates the two metal ions in a bridged disposition, drawing them inside the macrobicyclic cavity. The two metals were found to be quite isolated by the medium, and were coordinated by all the amine groups of L, as shown by the crystal structure of the dinuclear [Zn2H-1L]3+ species. This species can bind guests such as hydroxide and phosphate anions. Studies of anion binding in aqueous solution using pyrochatecol violet as the sensing guest revealed that the [Zn2H-1L]3+ species is able to bind one phosphate at physiological pH.